More info: Over the last few days the temps here have varied from mid 40s F to just over 70. I have noted the temps that our TCH displays in that range and their C conversion on the left, and on the right, as if the thermometer was in fact reading in C, and converting to F for display purposes:

Converting from F to C:........................ Converting from C to F
F ....... C............................................... C............F ... rounded/will display as
43........6.11 ............................................6......42.8......43
45........7.22 ............................................7......45.........45
46........7.77 ............................................8......46.4......46
48........8.88 ............................................9......48.2......48
50........10 ..............................................10......50........50
52........11.11 .........................................11......51.8.....52
54........12.22 .........................................12.......53.6....54
55........12.77..........................................13......55.4.....55
57........13.88..........................................14......57.2.....57
59........15 ............................................. 15......59........59
61........16.11 ..........................................16....60.8.......61
63........17.22 ..........................................17....62.6........63
64........17.77.......................................... 18.....64.4.......64
66........18.88 ..........................................19......66.2......66
68........20 ..............................................20......68.........68
70........21.11 ..........................................21....69.8.......70

So..It in fact appears that skata is right, the info is collected in degrees C and converted to F and then rounded to the nearest whole digit!